Short answer to all your questions is: "It depends" :)
Let's start with the last question, how much to get for 10 ppl. If you are cooking "bone in", you'll need a bigger PR as you'll lose weight to the bones. Additionally, it depends on whether or not the PR is trimmed, i.e. if the tail is taken off.
Here are two pics that demonstrate the difference.
Tail on:
Tail off (Rolled):
It might not seem like much, but if you are cutting 1+ inch slabs of PR, it could amount to 2-3 ounces per slice. For example, an 8lb lb untrimmed PR might drop to 6 pounds of desirable beef once the tail is trimmed off (if that is how you want it).
As to actual cook times, the first thing to consider is the internal temp of the PR when it goes into the smoker. IF you go straight from the fridge to the smoker, the cook time will be longer. If you bring the PR close to room temperature before putting it in the smoker, the cook time will be much less. This is one of the reasons that Bear's cook times are around 4 hrs, while my PR cooks in under 2 hours.
Something very important to point out is that you DON'T gauge PR cook times by the weight of the PR. What matters is how thick the roast is, not it's length. Say I have a 7 bone 14 pound PR. If I sectioned it off into a 3 bone 6lb roast and a 4 bone 8lb roast, each of those would take the same amount of time to cook. Additionally, either or both of them would take the same time to cook as the whole 7 bone 14lb roast would.
With all that said, here's some general rules of thumb.
If your PR goes into the smoker cold, it will take about 4 hours.
If your PR is close to room temp (68 degrees or so), it will take about 2 hours.
While it's cooking, monitor the internal temp (IT) with a good thermometer. If the IT is going up too fast, lower the temp of your smoker. If the PR is taking too long, turn the heat up to shorten the cook time. As long as you stay between 180 and 260, your PR should come out with a nice, even edge to edge color.
